It is already known from DE 4,242,293 that a belt pulley can be attached by means of a screw to the pivoting arm of a tensioning device. A sealing disk is clamped between the head of the screw and the lateral surface of the inner ring of the roller bearing to protect the roller bearing against the intrusion of dirt and other solid particles and to serve as a washer for the screw. When the belt pulley is fabricated and transported as a spare part for the known tensioning device or as an independent product for idling purposes, there is the danger that the accompanying screw and the sealing disk can fall out and be lost.